A nephelometric turbidimeter determines the concentration of solid particles or other particles suspended in a fluid which generally can be a liquid, a gas, or a mixture of liquid and gas. U.S. Pat. No. 8,724,107 B2 describes a nephelometric turbidimeter with a cylindrical turbidimeter vial comprising an optically transparent and flat bottom inlet window through which a measurement light beam axially enters the vial interior. A cylindrical section of the vial cylinder body defines a transparent outlet window through which the light scattered by the suspended particles radially exits the vial interior and is received by a light detecting means. The received primary light signal, which is caused only by the light scattered within the relevant fluid volume, is of relatively low intensity. Any light not caused by the light scattered within the relevant fluid volume but causing a secondary signal should therefore, if possible, be avoided.
The vial top opening is closed temporarily in case of a laboratory turbidimeter vial or is closed permanently in case of a process turbidimeter vial by a simple vial lid. If the turbidimeter vial is used to measure the turbidity of drinking water, the axial measurement light beam's intensity is reduced only minimally by the fluid turbidity so that the reflection of the measurement light beam at the vial lid's inside surface can cause a secondary light signal of high intensity which can be directly detected by the light detecting means. A secondary signal of high intensity could make it difficult, if not impossible, to determine the intensity of the primary light signal.